8. SUNYAEV-ZELDOVICH EFFECTS

The Sunyaev & Zeldovich (SZ) effect
(Sunyaev
& Zeldovich 1972)
arises from the inverse Compton scatter of CMB
photons against hot electrons. For a comprehensive background review see
Birkinshaw (1999).
The CMB intensity change is given by

The spectral form of this "thermal effect" is described by the function

(18)

which is negative (positive) at values of x smaller (larger) than
x0 = 3.83, corresponding to a critical frequency
0 = 217 GHz.

(19)

where me, ne and
Te are the electron mass, density and temperature
respectively,
T is the
Thomson cross section,
and the integral is over a line of sight through the plasma.

With respect to the incident radiation field, the change of the CMB
intensity across a galaxy or a cluster can be viewed as a net flux
emanating from the plasma cloud, given by the integral of intensity
change over the solid angle subtended by the cloud

(20)

In the case of hot gas trapped in the potential well due to an object of
total mass M, the parameter Y in eq. (20), called
integrated Y-flux, is proportional to the
gas-mass-weighted electron temperature <Te> and
to the gas mass Mg = fgM:

(21)

At frequencies below 217 GHz, the Y-flux is negative and can therefore
be distinguished from the positive signals due to the other source
populations.

Widely different formation modes for present day giant spheroidal
galaxies are being discussed in the literature, in the general framework
of the standard hierarchical clustering scenario. One mode
(Granato
et al. 2004,
Lapi et
al. 2006,
Cook et
al. 2009)
has it that these galaxies generated most of their
stars during an early, fast collapse featuring a few violent, gas rich,
major mergers; only a minor mass fraction may have been added later by
minor mergers. Alternatively, spheroidal galaxies may have acquired most
of their stars through a sequence of, mostly dry, mergers
(De
Lucia & Blaizot 2007,
Guo
& White 2008).

The second scenario obviously predicts far less conspicuous galaxy-scale
SZ signals that the first one. In the framework of the first scenario
(Massardi et al. 2008b)
find that the detection of substantial numbers of
galaxy-scale thermal SZ signals is achievable by blind surveys with next
generation radio telescope arrays such as EVLA, ALMA and SKA. This
population is detectable even with a 10% SKA, and wide-field-of-view
options at high frequencies on any of these arrays would greatly
increase survey speed. An analysis of confusion effects and
contamination by radio and dust emissions shows that the optimal
frequency range is 10-35 GHz. Note that the baryon to dark matter mass
ratio at virialization is expected to have the cosmic value, i.e. to be
about an order of magnitude higher than in present day
galaxies. Measurements of the SZ effect will provide a direct test of
this as yet unproven assumption, and will constrain the epoch when most
of the initial baryons are swept out of the galaxies.